The invention relates to material for electrical insulation which is made of a polyolefin composition with additives for preventing the formation of water trees. It is useful as insulation material especially in cables and wires for medium and high voltage such as about 10 kV and higher.
In electrically stressed polyolefin insulation, phenomenon can occur which is called "electrochemical treeing" (ECT) or "water treeing". This phenomenon, which is of importance particularly for the operating safety of plastic-insulated medium-and high-voltage cables, leads to the formation of tree-like structures in the insulation material, the so-called ECT structures.
The visual appearance of ECT structures which, after suitable staining are visible with particular contrast and detail, vary in size and extent. Basically, two forms can be distinguished:
"vented trees" which start at the surface of the insulation and extend into the insulation, and
"bow-tie trees" which are formed in the interior of the insulation.
The mechanism of the ECT formation has not been clarified at present. However, it is believed that an electric field and the presence of water are required for the formation of the ECT structures; hence, the ECT structures are also called "water trees". The points of initiation of the water trees always seem to be faults such as impurities, aggregated admixtures, cavities, gaps, cracks or boundary surfaces, of which, however, only a part leads to the formation of water trees. From the faults, which cannot be avoided completely in insulation produced on a large technical scale, the tree-like structures extend in the direction of the electric field.
Since ECT structures represent local changes of the insulating material, they can cause damage to the insulation, especially with respect to dielectric strength. Therefore, numerous attempts have been undertaken to prevent the growth of water trees or at least to slow it down.
One of the measures for preventing the formation of water trees is to provide the insulating layer with a metallic coating or jacket, for instance, of lead or aluminum. However, cables with such a water shielding layer are not only more expensive but also heavier and therefore, more difficult to handle than cables without metal jackets.
For this reason, prevention of the formation of ECT structures has been attempted by adding additives to the polymer of the insulating layer (or adjacent layers). Of the many compounds used in this connection, the following are exemplary: lead stearate (DE-OS No. 24 25 760 or British Pat. No. 1,473,867), sodium chloride and sulfate or other strong electrolytes (DE-AS No. 25 37 283 or U.S. Pat. No. 4,042,776), salts forming stable hydrates, such as calcium and magnesium chloride, and basic anhydrides (DE-OS No. 28 17 804 or British Pat. No. 1,584,501), silica gel and phosphorus pentoxide (DE-OS No. 27 54 336: page 6, last paragraph), organosilanes (U.S. Pat. Nos. 4,144,202; 4,212,756 and 4,263,158 as well as DE-OS No. 28 05 875 page 3, paragraphs 2 and 3), lead oxides and basic lead compounds (DE-OS No. 25 23 844: claim 1, and DE-OS No. 28 06 752: claims 1 and 3), organic isocyanates (U.S. Pat. No. 4,282,333), silane compounds grafted on the polymer material (DE-OS No. 29 35 224: claim 1) and metal complexes of diketones, salicylic acid or Schiff bases (European Pat. No. A1-27 300).
In contrast with the use of salt-like compounds or electrolytes, it has also been proposed to keep the content of finely distributed water-soluble and/or hygroscopic salts in the insulation below a value of 10.sup.-1 ppm and preferably, below 10.sup.-4 ppm (DE-OS No. 29 11 756: page 3, paragraphs 3 and 4).
These measures, which are not consistent, have not generally produced the desired results. For example, a report by Cigre, the International High-Voltage Conference, of 1980 (see "Kunststoffe" (Plastics) 71, 1981, pages 448 et seq.) recommends that a watertight metal shield be applied to high voltage cables in order to prevent moisture from penetrating.
It is, therefore, an object of the invention to prevent effectively and permanently the formation of water trees in electrical insulation for medium and high voltage from about 10 kV up by use of suitable additives. A further object is to avoid expensive measures such as metal jackets.